1. Field of the Invention
The present invention relates in general to a device for producing apodized gratings.
2. Description of the Related Art
Apodized gratings are particularly important for telecommunications applications, such as dense wavelength division multiplexing and dispersion compensation, because well-designed apodization profiles can considerably suppress the sidelobes in the grating reflection spectra. J. Martin et al. disclosed a method of producing apodized gratings (ELECTRONICS LETTERS, May 12, 1994, Vol.30, No.10, pp.811-812). Referring to FIG. 1, an optical fiber 11 and a phase mask 12 are movably mounted on a translation stage 13, wherein the optical fiber 11 is placed behind the phase mask 12. Then, a UV beam 14 is projected through the phase mask 12 to write gratings on the optical fiber 11. In this method, however, any offset of the beam position relative to the optical fiber center during the translation results in a corresponding variation of the average refractive index of the optical fiber. The variation of the average refractive index imparts an often undesired chirp to the gratings. To resolve such a problem, W. H. Loh et al. disclosed a moving fiber-scanning beam technique (OPTICS LETTERS, Oct. 15, 1995, Vol.20, No.20, pp.2051-2053). Referring to FIG. 2, an optical fiber 21 is mounted on a computer-controlled stage 23 which slowly moves with respect to a phase mask 22. A UV beam 24 is reflected by a moving mirror 25 to pass through the phase mask 22 so as to write gratings on the optical fiber 21. By the movement of the UV beam, variation of the average refractive index of the optical fiber is totally avoided because the average UV fluence along the length of the grating is the same. However, simultaneously control of the movement of the stage 23 and the mirror 25 is required in this system. Such a system is very complex.